1. Field of the Invention
The present invention relates to an image scanning apparatus, particularly, the invention is preferably applied to an image scanning apparatus having a function of scanning a flowing image of a document fed at predetermined speed.
2. Description of the Related Art
In related arts, the following two systems are well known as systems adopted in image scanning apparatus for scanning an image on a document.
First, a first system is a fixed document scanning system for scanning an image on a document by an image scanning portion by setting the document on a document base of an image scanning apparatus by a predetermined method and scanning the document fixed onto a reference document base glass by a movable optical carriage.
Further, a second system is a flowing document scanning system fox scanning the image on the document by feeding the document at predetermined feed speed by a predetermined scanning portion arranged with a fixed optical carriage portion.
According to the later of the flowing document scanning system, the optical carriage can be fixed and the optical carriage needs not to reciprocate at respective times. Therefore, according to the flowing document scanning system, an apparatus constitution is not complicated in comparison with the former of the fixed document scanning system.
Further, the flowing document scanning system is superior to the former of the fixed document scanning system also in view of high speed formation of processings.
Further, by using the image scanning systems in a copier, even when a plurality of sheets of copies are formed from the same document, the document is scanned only by one time. This is because according to a copier having a scanning function, it can easily be carried out to temporarily hold image information which is scanned once at a storage medium of a computer or the like as digital information and output again a necessary number of sheets of copied images based on the stored image information. Further, by using the above-described flowing document scanning system in the copier, the high speed formation of processings or simplification of a structure of the copier per se can be promoted.
Further, there is an image scanning apparatus for scanning images on two faces of a document including the images on a top side and a bottom side thereof. Such an image scanning apparatus is constituted to provide two optical carriages for scanning the images included in the respective faces of the document by the respective optical carriages provided along a feed path of the document from a view point of achieving convenience and high speed formation of operation.
Therefore, according to the apparatus constitution, the flowing images on the top side and on the bottom side of the document fed along the predetermined path can be scanned substantially simultaneously and therefore, high speed formation of a scanning processing in copying the two faces of the document can be achieved.
However, according to the above-described image scanning apparatus of the flowing document scanning system, a disturbance in a behavior of the document in scanning effects adverse influence on scanning accuracy. The disturbance of the behavior of the document in scanning is brought about by a change in feed speed of the document or upward and downward movement of the document in the feed path to cause to elongate or contract the scanned image or disturb the image abruptly. Further, when the image scanning apparatus of the flowing document scanning system is a color scanner, respective colors are scanned by elements of scanning the respective colors aligned in a document feed direction at timings in compliance with the feed speed of the document and electrically overlapped to be output again as a color image. Therefore, the colors of the scanned image are shifted to emerge by disturbing the behavior of the document in feeding.
From such a reason, high accuracy is requested in feeding the document in the image scanning apparatus of the flowing document scanning system. In order to meet the request, there is a flowing document scanning system for driving rollers for feeing a plurality of documents by separate motors and enabling to set the feed speed of the document further finely.
According to the image scanning apparatus of the flowing document scanning system, dispersion in a dimension of a roller for feeding the document or integration thereof or a change in an outer shape by an ageing change of the roller can be adjusted for each machine.
Further, according to the above-described image scanning apparatus of the flowing document scanning system, it is possible to set to stabilize feeding of the document by producing an arbitrary speed difference between the feed speeds of the respective rollers.
However, by producing the speed difference between the respective rollers, there poses a problem that depending on by which roller the document is fed or at which position the document is fed, the scanning speed of the document is changed in scanning or the behavior or the feed speed of the document is changed at an instance of charging the document to the roller or at an instance of separating a rear end thereof from the roller to thereby bring about disturbance or color shift of the image.